Description of the Prior Art
In the conventional method of recording information onto an optical information recording medium (hereinafter abbreviated to "recording medium") and playing back information from that recording medium, an optical head of an optical-system construction is used, such as that shown in FIG. 14. The operation of the optical system of the optical head of FIG. 14 is described below. First, a divergent beam of light 2 emitted from a semiconductor laser 1 is converted by a collimator lens 3 into a parallel beam of light 4, and the parallel beam of light 4 is incident on a diffraction grating 5 and is converted thereby into three transmission diffraction beams of light at dimensions 0 and .+-.1.
These three transmission diffraction beams of light, in other words, three light beams 6, are incident on a beam splitter 7 and are both passed and reflected by a semi-transparent diaphragm 8 of the beam splitter 7. Three beams of light 9 that are passed by the beam splitter 7 are focused by a focusing lens 10 into three very small spots of light 11 (hereinafter abbreviated to "light spots") arranged on a straight line, and are shone onto a recording medium 12. In this case, a high-output beam of light from the semiconductor laser 1 modulated by an information signal is used to record information, and a continuous low-output beam of light is used to play back the information. The central light spot of the three light spots 11 arranged in a straight line is a main light spot that is a 0-dimensional diffraction light spot used for recording and playing back information and for focusing, and the two auxiliary spots of light positioned on either side thereof are .+-.1-dimensional diffraction spots of light used for tracking.
The three beams of light shone onto the recording medium 12 as the three light spots 11 are partially reflected thereby, and the resultant three reflected beams of light are passed back through the focusing lens 10 which converts them back into approximately parallel beams of light. These beams of light are reflected by the semi-transparent diaphragm 8 of the beam splitter 7 to the right in the figure, are condensed by a photoreceiver lens 13, and reach a photoreceiver diode 15 through a cylindrical lens 14. An information signal, a focusing control signal that indicates the focusing status of the light spots 11, and a tracking control signal that indicates the positional status between a track provided on the recording medium 12 and the light spots 11 are obtained from the light-receiving diode 15. Note that the photoreceiver lens 13 is an optical element that acts to condense the three reflected parallel beams of light, and the cylindrical lens 14 is an optical element in which astigmatism occurs in order to provide a focusing control signal. Thus, the optical head shown in FIG. 14 uses an astigmatism method to derive a focusing control signal and a three-beam method to derive a tracking control signal.